Ethyl butyrate Figure 21. Esters with specific fruit flavors.
In general, the most abundant alcohol in the volatile fraction of a fruit is usually present in the most abundant ester. Aromatic aldehydes sometimes constitute the main flavor component of certain fruits. Benzaldehyde (Fig. 22), for example, is the principal flavor compound in bitter almond. It is also present in the volatiles of peaches, apricots, cherries, and plums. They are generally produced from cy-anogenic glycosides by the action of /?-glucosidases, which hydrolyzes the glycoside into the free sugar and (R)-mandelonitrile. The latter then undergoes an elimination reaction in the presence of hydroxynitrile lyase to produce the benzaldehyde. 4-Hydroxy-3-methoxybenzaldehyde (vanillin), another aromatic aldehyde, is also produced by the same series of enzymes.
Origins of Enzymatically Produced Flavors in Dairy Products: Milk and Cheese
In contrast to vegetables and fruits, in which the endogenous enzymes are responsible for the release of aroma compounds, dairy flavors are generated by in addition to endogenous enzymes, added microbial enzymes that play a predominant role in the formation of flavors. In general, the flavor of dairy products originates from microbial, enzymatic, and chemical transformations (mostly oxidations)—the relative importance of which is not always understood. These transformations give rise to a series of volatile and nonvolatile compounds, some of which have been shown to correlate well with some typical dairy flavor notes and some have little indication of their real contri-
Figure 22. Benzaldehyde.
bution. For any dairy aroma, the total number of compounds identified is far less than in those foods subjected to thermal treatment. Thousands of heterocyclic compounds present in such foods are missing in fermented ones. The characteristic dairy food flavors are usually related to the occurrence of relatively few key components.
Milk furnishes the flavor precursors for all the fermented products (cheese, yogurt etc) made from it. The milk lactose, in addition to being a nutrient for the growth of the starter microorganisms, is also the principal precursor of 2,3-butadione (diacetyl), an important constituent of cultured products. It imparts characteristic creamy note to most cheeses and other dairy products. It is found in butter, unripened soft cheeses such as cottage cheese, and also in Swiss cheese. Milk proteins (caseins) are the source of bitter peptides that contribute to the aroma of different cheeses. The sweet flavor of Swiss cheese is due to a complex formed between calcium (or magnesium) and peptides. The burned note of Gruyère cheese is attributed to peptides complexed with 2,5-dimethyl-4-hydroxy-3-[2ifJfuranone; these peptides and amino acids are produced during the ripening of cheese by the action of proteolytic enzymes on the milk caseines. Some amino acids can be oxidized into corresponding carboxylic acids, such as alanine, and into propionic acid, which is responsible in part for the aroma and taste of Swiss cheese. Although carbohydrates and proteins play an important role as precursors of cheese aroma, fatty acids are the key precursors for both desirable and undesirable aroma. C4-C10 fatty acids impart well-known pungent notes to the cheese, whereas 2-methylketones, lactones, and dairy-associated aldehydes such as cis-4-heptenal are important precursors to other flavors. In addition, they act as a medium for the action of enzymes responsible for the formation of aroma, and they serve as a repository for all the other aroma compounds. As such, they determine the relative composition of the aroma vapor above the cheese.
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